It is well known that adhesion to the surfaces of materials is improved by the exposure to a corona discharge. Thus corona treatment has been used to treat the surfaces of thermoplastic materials to improve the adhesion of printing inks, paints, coatings and bodies of other materials.
Many methods for the continuous corona discharge surface treatment of thermoplastic materials have been employed wherein the material continuum is passed through an air gap between stationary and roller electrodes. The stationary electrode is typically a bar or cluster of bars and the relatively large roller electrode is coated with a dielectric coatings. A high voltage, of the order of 20 KV at 10 kHz, is typically impressed across the electrodes. A corona arc discharge is developed in the gap and produces surface treatment of the thermoplastic material continuum which results in the promotion of excellent adhesion properties on the surface of the treated continuum. Recently, the stationary electrode has been coated with a dielectric layer rather than the roller electrode.
The watts per inch of the corona discharge system during the continuous treatment of a thermoplastic material continuum results in the temperature of the surface of the electrode containing the dielectric coating to increase to a temperature as high as 150.degree. C. Consequently, at this high temperature, the dielectric layer and the substrate will expand and if their coefficients of thermal expansion is not relatively close then the dielectric layer could crack, bend, distort or spall off. To overcome this problem, the substrate material of the electrode and the material of the dielectric layer are selected so that their coefficients of thermal expansion were relatively close. For example, U.S. Pat. No. 4,841,409 discloses a method of manufacturing a corona discharge electrode at which the core material of the electrode has approximately the same temperature expansion coefficient as the dielectric material employed as the coating.
Cooling means are also used to keep the temperature of the dielectric coated electrode stable and within control. In U.S. Pat. No. 4,145,386 a cylindrical electrode is shown that is cooled by air. In U.S. Pat. No. 4,334,144 a corona apparatus is shown in which the inside of a coated corona electrode is cooled by a fluid. Specifically, in one embodiment the electrode comprises a tubular conductor of copper placed within a glass tube and then a dielectric fluid of oil is fed through the copper tube to cool the electrode. In this embodiment, glass is the dielectric layer of the electrode.
Copper is a good conductor but does not have a coefficient of thermal expansion that is close to a dielectric coated layer such as alumina. Thus when using an elongated alumina coated copper corona electrode in a corona process, the surface of the electrode facing the other electrode of the corona system will rapidly increase in temperature. The difference in temperature at the surface of the electrode facing the other electrode of the corona system will rapidly increase in temperature. The difference in temperature at the front surface of the electrode can become significantly higher than the back surface of the copper electrode which could result in distortion of the electrode. If the distortion is severe, the dielectric layer could crack and spall off. At a minimum, a distortion of the electrode could result in a non-uniform corona discharge along the length of the electrode due to a variation in the spacing between the electrodes that could affect the quality of the corona treatment of the thermoplastic sheet being treated.
It is an object of the present invention to provide a process for the corona surface discharge treatment of thermoplastic materials which uses a hollow elongated dielectric coated copper electrode that is cooled using a fluid such as water.
It is another object of the present invention to provide a hollow elongated dielectric coated copper electrode that is water cooled.
It is another object of the present invention to provide a hollow elongated dielectric coated copper electrode that is fluid cooled and thus enable the electrode to operate at a lower temperature at high power levels and high speed operations without distortion of the electrode.